Fuel cell anode structures for voltage reversal tolerance

ABSTRACT

An improved fuel cell anode structure comprises a substrate and a first carbon-based component. The first carbon-based component exhibits little or no resistance to corrosion. When said anode structure is incorporated into a membrane electrode assembly, the membrane electrode assembly is tolerant to incidences of cell voltage reversal.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/835,905 filed Apr. 16, 2001, entitled “Fuel Cell AnodeStructures For Voltage Reversal Tolerance”. The '905 application is, inturn, a continuation-in-part of U.S. patent application Ser. No.09/585,696 filed Jun. 1, 2000 (now U.S. Pat. No. 6,517,962 issued Feb.11, 2003), entitled “Fuel Cell Anode Structures For Voltage ReversalTolerance”. The '905 application also related to and claimed prioritybenefits from PCT/International Application No. PCT/GBOl/00458 filedFeb. 6, 2001. The '696 application, in turn, related to and claimedpriority benefits from U.S. Provisional Patent Application Serial No.60/150,253 filed Aug. 23, 1999. The '905, '696, '253 and '458applications are each hereby incorporated by reference herein in theirentireties.

FIELD OF THE INVENTION

[0002] The present invention relates to an anode structure comprising asubstrate and a first carbon-based component that shows little or noresistance to corrosion, such that when the anode structure isincorporated into a membrane electrode assembly, the membrane incidencesof cell voltage reversal.

BACKGROUND OF THE INVENTION

[0003] A fuel cell is an energy conversion device that efficientlyconverts chemical energy into electrical energy by electrochemicallycombining either hydrogen, normally stored as a gas, or methanol,normally stored as a liquid or gas, with oxygen, normally in the form ofair, to generate electrical power. At their fundamental level, fuelcells comprise electrochemical cells formed from a solid or liquidelectrolyte and two electrodes, the anode side and cathode side, atwhich the desired electrochemical reactions take place. In the fuelcell, the hydrogen or methanol is oxidized at the anode side and theoxygen is reduced at the cathode side to generate the electrical power.

[0004] Normally in fuel cells the reactants are in gaseous form and arediffused into the anode and cathode structures. The electrode structuresare therefore specifically designed to be porous to gas diffusion inorder to facilitate contact between the reactants and the reaction sitesin the electrode to promote the reaction rate. Efficient removal of thereaction products from the electrode structures is also important. Incases where liquid reactants and products are present the electrodestructures are often tailored to efficiently feed reactants to andremove products from the reaction sites. The electrolyte is also incontact with both electrodes and in fuel cell devices may be acidic oralkaline, as well as liquid or solid in nature.

[0005] The proton exchange membrane fuel cell (PEMFC) is the likely typeof fuel cell to find wide application as an efficient and low emissionpower generation technology for a range of markets, such as in a rangeof stationary, residential and portable power generation devices and asan alternative to the internal combustion engine for transportation. Inthe PEMFC, whether hydrogen or methanol fuelled, the electrolyte is asolid proton-conducting polymer membrane, commonly based onperfluorosulfonic acid materials.

[0006] In the PEMFC, the combined laminate structure formed from themembrane and the two electrode structures is known as a membraneelectrode assembly (MEA). The MEA typically comprises several layers,but in general can be considered to comprise five layers that arecharacterized by their function. On either side of the membrane an anodeelectrocatalyst or a cathode electrocatalyst is incorporated to increasethe rates of the desired electrode reactions. In contact with theelectrocatalyst containing layers, on the opposite face to that incontact with the membrane, are the anode and cathode gas diffusionlayers. The anode gas diffusion layer is typically porous to allow thereactant hydrogen or methanol to enter from the face of the layerexposed to the reactant fuel supply. The reactant then diffuses throughthe thickness of the gas diffusion layer to the layer containing theelectrocatalyst, which is usually platinum metal based, to facilitatethe electrochemical oxidation of hydrogen or methanol. The anodeelectrocatalyst layer also typically comprises some level ofproton-conducting electrolyte in contact with the same electrocatalystreaction sites. With acidic electrolyte types, the product of the anodereaction is protons, and the protons are then transported from the anodereaction sites through the electrolyte to the cathode layers. Thecathode gas diffusion layer is also typically porous to allow oxygen orair to enter the layer and diffuse through to the electrocatalyst layerreaction sites. The cathode electrocatalyst facilitates the chemicalcombination of the protons with oxygen to produce water, and alsotypically comprises some level of the proton-conducting electrolyte incontact with the same electrocatalyst reaction sites. Product water thendiffuses out of the cathode structure. The structure of the cathode isnormally designed to enable efficient removal of product water. If waterbuilds up at or in the cathode, it becomes more difficult for thereactant oxygen to diffuse to the reactant sites, and thus theperformance of methanol-fuelled PEMFCS, additional water is present dueto the water contained in the methanol, which can be transported throughthe membrane from the anode to the cathode side. The increased quantityof water at the cathode requires additional water removal capabilities.However, it is also the case with proton-conducting membraneelectrolytes, that if too much water is removed from the cathodestructure, the membrane can dry out, thereby resulting in a significantdecrease in the performance of the fuel cell.

[0007] The complete MEA can be constructed by several methods. Theelectrocatalyst layers can be bonded to one surface of the gas diffusionlayer to form what is known as a catalyzed gas diffusion layer or gasdiffusion electrode. Two gas diffusion electrodes can be combined withthe solid proton-conducting membrane to form the MEA. Alternatively, twoporous uncatalyzed gas diffusion layers can be combined with a solidproton-conducting polymer membrane that is catalyzed on both sides toform the MEA. Further, one gas diffusion electrode can be combined withone uncatalyzed gas diffusion layer and a solid proton-conductingpolymer membrane that is catalyzed on the side facing the gas diffusionlayer to form the MEA.

[0008] The materials typically employed in the fabrication of theuncatalyzed gas density materials such as rigid carbon fiber paper (suchas, for example, Toray TGP-H-60 or TGP-H-90 from Toray Industries,Japan) or woven carbon cloths (such as Zoltek PWB-3 from ZoltekCorporation, 3101 McKelvey Road, St. Louis, Mo., USA 63044). Layers suchas these are usually modified with a particulate material eitherembedded within the fiber network or coated on to the large planarsurfaces, or a combination of both. Typically, these particulatematerials comprise a carbon black and polymer mix. The particulatecarbon black material is, for example, an oil furnace black (such asVulcan XC72R from Cabot Chemicals, Billerica, Ma, USA) or an acetyleneblack (such as Shawinigan from Chevron Chemicals, Houston, Tex., USA).The polymer most frequently employed is polytetrafluoroethylene (PTFE).The coating, or embedding, is carried out in order to improve the watermanagement properties, improve gas diffusion characteristics, to providea continuous surface on which to apply the catalyst layer and to improvethe electrical conductivity. More recently, electrode structures basedon gas diffusion layers comprising a non-woven network of carbon fibers(carbon fiber structures such as Optimat 203 from Technical FiberProducts, Kendal, Cumbria, UK) with a particulate material embeddedwithin the fiber network, as disclosed in European Patent PublicationNo. 0791974, have shown comparable performances to structures based oncarbon fiber paper or cloth.

[0009] The electrocatalyst materials for the anode and cathodestructures typically comprise precious metals, in particular platinum,as these have been found to be the most efficient and stableelectrocatalysts for low-temperature fuel cells such as the PEMFC.Platinum is employed either on its own as the only electrocatalyticmetal or in combination with other precious metals or base metals. Theplatinum-based electrocatalyst is provided as very small particles(approximately 20-50Å in diameter) of high surface area, which areusually distributed on and supported by larger macroscopic conductingcarbon particles to provide a desired catalyst loading. Conductingcarbons are the preferred materials to support the catalyst. Particulatecarbon black materials typically employed include Vulcan XC72R andShawinigan. It is also possible to employ a platinum-basedelectrocatalyst that does not incorporate a support, and in this case itis referred to as an unsupported Pt electrocatalyst.

[0010] Each MEA in the PEMFC is sandwiched between electricallyconducting flow field plates that are conventionally based upon carbonand contain channels that feed the MEA with the reactants and throughwhich the products are removed. Since each MEA typically delivers0.6-0.7 V, usually between 10 to 100 such MEAs are each interposedbetween flow field plates to form stacks. These stacks are combinedelectrically in series or parallel to produce the desired power outputfor a given application.

[0011] Recently, it has been observed that during prolonged operationsome cells in large stacks can go into an undesired condition known ascell voltage reversal or, simply, cell reversal. This is shown by thecell potential becoming negative rather than the positive valueassociated with normal PEMFC operation. Such cell reversals can be dueto depletion in the concentration of the reactants at the cathode oranode sides, which can be caused by a number of factors such asrestricted gas flow due to blocked flow fields or poor waterdistribution in the MEA. In combination with this, especially insituations in which a fast dynamic response is required, such as intransportation applications, it is possible that the gas flow cannotrespond quickly enough to sustain the current demand. Further, if onecell in a stack shows cell reversal, adjacent cells in the stack mayalso overheat, resulting in cell reversal.

[0012] If the cell reversal is due to a restricted oxygen concentrationat the electrocatalyst sites in the cathode then, to sustain the flow ofcurrent, hydrogen is produced at the cathode,

2H⁺+2e ⁻→H₂

[0013] Since hydrogen production at the cathode is very facile at theplatinum-based electrocatalysts typically employed, the electrodepotential is usually only slightly more negative than that for hydrogenoxidation at the anode. The result is that at normal operating currentdensities the cell voltage is normally slightly negative, for example,−0.1 V. This type of cell reversal raises safety and durabilityconcerns, since hydrogen is being produced in the oxidant side of thecell, a significant quantity of heat is generated, and water is nolonger being produced at the cathode. Such product water helps tosustain membrane hydration, especially at the membrane-anode interface,since it promotes the back-diffusion of water.

[0014] A major problem occurs, however, if the hydrogen concentration isrestricted at the anode side. In this case to sustain the flow ofcurrent water electrolysis and carbon corrosion can occur, as follows:

2H₂O→O₂+4H⁺+4e ⁻

C+2H₂O→CO₂+4H⁺+4e ⁻

[0015] Since both electrode reactions occur at more positive electrodepotentials than oxygen reduction at the cathode, again, the cell voltageis negative, but in this case the cell voltage may be as high as −0.8 Vat typical operating current densities. While carbon corrosion isfavored over water electrolysis thermodynamically, the electrochemicalkinetics of water electrolysis are sufficiently facile at theplatinum-based electrocatalysts typically employed in the PEMFC thatinitially water electrolysis principally sustains the current. There isonly a small contribution from corrosion of the carbon components in theanode to the cell current. If, however, the anode catalyst becomesdeactivated for water electrolysis or if the water concentration at theelectrocatalyst sites in the anode becomes significantly depleted, thewater electrolysis current is gradually replaced by increased rates ofcarbon corrosion. In the case of carbon corrosion, water need only bepresent in the vicinity of the relevant, abundant carbon surfaces.During this period the cell voltage becomes more negative (that is, theanode potential becomes more positive) to provide the necessary drivingforce for carbon corrosion. This in turn increases the driving force forthe water electrolysis reaction. The result, if such cell reversal isprolonged, may be irreversible damage to the membrane and catalystlayers due to excessive dehydration and localized heating. Further, thecatalyst carbon support in the anode structure corrodes, with eventualdissolution of the platinum-based catalyst from the support, and theanode gas diffusion layer may become degraded due to corrosion of thecarbon present in the gas diffusion layer structure. In cases where thebipolar flow field plates are based upon carbon the anode flow fieldplate may also be subjected to significant carbon corrosion, therebyresulting in surface pitting and damage to the flow field pattern.

[0016] It would therefore be a significant advantage to protect the MEAfrom the effects of cell reversal should a cell go into cell reversal.

SUMMARY OF THE INVENTION

[0017] An anode structure for a proton exchange membrane fuel cell(PEMFC) comprises a substrate and a first carbon-based componentcomprising a first carbon material. The first carbon-based componentexhibits little or no resistance to corrosion. When the present anodestructure is incorporated into a membrane electrode assembly, the MEA issubstantially tolerant to incidences of cell reversal.

[0018] The term “anode structure” in the context of the presentspecification means any of the functional components and structuresassociated with the anode side of the MEA through which a fuel is eithertransported or reacted, that is, within the gas diffusion andelectrocatalyst containing layers on the anode side of the membrane. Thepractical embodiments of the present anode structure as herein definedinclude:

[0019] (a) a gas diffusion layer;

[0020] (b) an electrocatalyst containing layer bonded to a gas diffusionlayer (also referred to as a gas diffusion electrode or acatalyst-coated gas diffusion layer);

[0021] (c) an electrocatalyst containing layer bonded to theproton-conducting membrane (also referred to as a catalyst-coatedmembrane).

[0022] In the context of the present specification, the term “substrate”refers to a gas diffusion layer or a polymer membrane electrolyte.

[0023] The first carbon-based of the present anode structure componentmay consist entirely of a first carbon material or may comprise a firstcarbon material and one or more other materials that may for example bepresent to promote the corrosion rate of the first carbon material or toact as a binder. The one or more other materials that may be present inthe first carbon-based component include polymeric materials such as,for example, a proton-conducting polymer electrolyte, such as Nafion®,or a non-proton-conducting polymer such as, for example,polytetrafluoroethylene (PTFE). The first carbon-based component presentin the anode structure (whether solely of first carbon material or offirst carbon material plus other material(s)) shows little or noresistance to corrosion, and therefore when used in an electrochemicalcell that has entered a period of cell reversal, the first carbon-basedcomponent will be corroded in preference to other carbon also present inthe anode structure, for example a carbon support for theelectrocatalyst. In other words, the first carbon-based component isacting as a sacrificial carbon component. This will protect furthercarbon present in the anode from corrosion and thus maintain its desiredfunction when the cell returns to normal operation. For instance, thiswill inhibit the carbon black in the electrocatalyst carbon support andthe carbon in the gas diffusion layer from corroding. Consequently, theanode electrocatalyst and the anode gas diffusion layer will beprotected from the effects of cell reversal, thereby allowing the cellto function without having suffered significant irreversible performancedecay when the cell reverts to normal fuel cell operation after the cellreversal incident. To promote the corrosion rate of the first carbonmaterial used in the first carbon-based component, the first carbonmaterial may be pre-treated with a suitable form of theproton-conducting membrane electrolyte prior to incorporation into theanode structure. Impregnating the first carbon material withproton-conducting membrane electrolyte will promote the corrosion rateof the first carbon-based component by providing an efficient conductionpathway for the protons formed in the carbon corrosion reaction to themembrane of the MEA.

[0024] Further, the first carbon-based component allows the membrane andcatalyst layer in the MEA to function without having sufferedsignificant irreversible performance decay when the cell reverts tonormal fuel cell operation after the cell reversal incident. This isbecause corrosion of the first carbon-based component helps sustain thecurrent density at a less negative cell voltage, corresponding to a lesspositive anode potential. At less positive anode potentials the drivingforce for irreversible damage to the membrane and catalyst layers isreduced.

[0025] As a general rule, the corrosion resistance of carbons is relatedto the degree of the graphitic nature within the structure. The moregraphitic the structure of the carbon the more resistant the carbon isto corrosion. The typical carbon blacks employed in fuel cells, eitheras the electrocatalyst support or in the gas diffusion layer, thereforetend to be those that are more highly graphitic in nature as theenvironment particularly at the cathode is very oxidising. It iscontemplated that the first carbon material will be chosen from thegroup of carbons that are much less graphitic, that is, more amorphousthan the typical carbon materials employed in the fuel cell.

[0026] In a further embodiment of the present anode structure, the anodestructure further comprises a second carbon component that issubstantially more resistant to corrosion than the first carbon-basedcomponent. For example, the second carbon component may be a carbonsupport for an electrocatalyst or a carbon fill for a gas diffusionsubstrate.

[0027] In embodiments of the present anode structure, a gas diffusionlayer may comprise a first carbon-based component. The firstcarbon-based component may either be embedded within the gas diffusionlayer or applied as a coating to one or both surfaces, or a mixture ofboth. To prepare a gas diffusion layer according to the presenttechnique, the first carbon-based component may be mixed with a carbonblack filler material typically employed to coat or fill the carbonpaper, cloth or non-woven fiber web substrates employed in the PEMFC toproduce the anode structure of the invention in the form of a gasdiffusion layer. To promote the corrosion rate of the first carbonmaterial used in the first carbon-based component, the first carbonmaterial may be pre-treated or the resultant anode gas diffusion layersubsequently treated with a suitable form of the proton-conductingmembrane electrolyte prior to incorporation in the MEA. The carbon blackfiller material usually comprises a particulate carbon and a polymer,the carbon suitably being in the form of a powder. The carbon powder maybe any of the materials generally designated as carbon black, such asacetylene blacks, furnace blacks, pitch coke based powders andgraphitised versions of such materials. Suitably, both natural andsynthetic graphites may also be employed in this application. Suchmaterials may be employed either alone or in combination. Theparticulate carbon, or carbons, in the fill are held together by one ormore polymers. The polymeric materials employed contribute to theelectrode structural properties, such as pore size distribution,hydrophobic/hydrophilic balance and physical strength of the gasdiffusion layer. Examples of such polymers include PTFE, fluorinatedethylene-propylene (FEP), polyvinylidene difluoride (PVDF), Viton A,polyethylene, polypropylene, ethylene-propylene. The preferred polymeris PTFE or FEP.

[0028] In addition other modifier materials and catalyst materials,which are not electro-catalysts, may be added to the carbon black fillersuch as disclosed in PCT/International Publication No. WO 00/55933(Johnson Matthey).

[0029] Furthermore, the first carbon-based component may be applied toan anode gas diffusion layer that has previously been coated or filledwith typical carbon filler materials. To promote the corrosion rate ofthe first carbon material employed in the first carbon-based component,it may be pre-treated with the suitable form of the proton-conductingmembrane electrolyte. It is contemplated that in the MEA formed usingthe resultant anode gas diffusion layer, the first carbon-basedcomponent within the anode layer may face either the electrocatalystlayer or the anode flow field plate. In the present anode structure, theanode gas diffusion layer should have sufficient electrical conductivitysuch that on removal of the first carbon-based component during cellreversal, the remaining anode layer does not have a significantly lowerelectrical conductivity. Typical substrates that could be employedinclude those based upon Toray carbon fiber paper and Zoltek PWB-3carbon cloth, which without a carbon coating or fill have through planespecific electrical resistivities of below 0.15 Ωcm.

[0030] In another embodiment of the anode structure, a gas diffusionelectrode comprises a first carbon-based component. The firstcarbon-based component may be admixed with an electro-catalyst componentand a polymeric material and the two applied to a gas diffusion layer asa single admixed layer, or the first carbon-based component and theelectrocatalyst component may be applied as separate layers, eachseparate layer also incorporating a polymeric material. Alternatively,there could be a combination of separate and mixed layers. The polymericmaterial may be a soluble form of the proton-conducting membraneelectrolyte, or may be any of a wide range of polymeric materials usedto contribute to the structural and diffusional properties. Examples ofsuch polymers include PTFE, FEP, PVDF, Viton A, polyethylene,polypropylene, ethylene-propylene. The preferred polymer is PTFE or FEP.To promote the corrosion rate of the first carbon material used in thefirst carbon-based component the first carbon material may bepre-treated with a suitable form of the proton-conducting membraneelectrolyte prior to incorporation into the anode electro-catalystmixture.

[0031] The mixture of first carbon-based component and anodeelectrocatalyst can be deposited onto the typical range of gas diffusionlayers employed in PEMFCs to produce the anode structure of theinvention in the form of a gas diffusion electrode.

[0032] Typical anode electrocatalysts employed in the PEMFC may be, forexample, a precious metal or a transition metal as the metal or metaloxide, either unsupported or supported in a dispersed form on a carbonsupport; an organic complex, in the form of a high surface area finelydivided powder or fiber, or a combination of these options. An exampleof a suitable electrocatalyst material is described in European PatentPublication No. 0731520. Particularly suitable electrocatalysts areunsupported platinum (Pt) or alloys or mixtures of platinum/ruthenium(PtRu) and carbon supported Pt or PtRu. The PtRu electrocatalystexhibits a higher level of tolerance to CO and CO₂ when they are presentin the fuel stream than Pt electrocatalysts.

[0033] Specific examples of this embodiment may be prepared according tothe procedure described in more detail hereinbelow.

[0034] In another embodiment of the anode structure, a catalyst-coatedmembrane comprises a first carbon-based component. The firstcarbon-based component may be admixed with an electrocatalyst componentand a polymeric material and the two applied to a membrane electrolyteas an admixed single layer, or the first carbon-based component and theelectrocatalyst component may be applied as separate layers, eachseparate layer also incorporating a polymeric material. Alternatively,there could be a combination of separate and mixed layers. The polymericmaterial may be a soluble form of the proton-conducting membraneelectrolyte, or may be any of a wide range of polymeric materials usedto contribute to the structural and diffusional properties. Examples ofsuch polymers include PTFE, FEP, PVDF, Viton A, polyethylene,polypropylene, ethylene-propylene. The preferred polymer is PTFE or FEP.To promote the corrosion rate of the carbon in the first carboncomponent, the carbon may be pre-treated with a suitable form of theproton-conducting membrane electrolyte prior to incorporation into theanode electrocatalyst mixture.

[0035] The mixture of first carbon-based component and anodeelectrocatalyst can be deposited onto the solid membrane electrolyte toproduce the anode structure of the invention in the form of acatalyst-coated membrane. Subsequent compression of the anode andcathode catalyst-coated membrane to the typical gas diffusion layersemployed in PEMFCs, or hot pressing of anode and cathode catalyst-coatedgas diffusion layers to the solid proton-conducting membrane electrolyteforms the complete MEA.

[0036] Typical anode electrocatalysts employed in the PEMFC are aspreviously described.

[0037] The proton-conducting polymers suitable for use in the presentanode structure may include, but are not limited to:

[0038] (a) Polymers which have structures with a substantiallyfluorinated carbon chain optionally having attached to it side chainsthat are substantially fluorinated. These polymers contain sulfonic acidgroups or derivatives of sulfonic acid groups, carboxylic acid groups orderivatives of carboxylic acid groups, phosphonic acid groups orderivatives of phosphonic acid groups, phosphoric acid groups orderivatives of phosphoric acid groups and/or mixtures of these groups.Perfluorinated polymers include Nafion®, Flemion® and Aciplex®commercially available from E. I. DuPont de Nemours (U.S. Pat. Nos.3,282,875; 4,329,435; 4,330,654; 4,358,545; 4,417,969; 4,610,762;4,433,082 and 5,094,995), Asahi Glass KK and Asahi Chemical Industryrespectively. Other polymers include those disclosed in U.S. Pat. Nos.5,595,676 (Imperial Chemical Industries plc) and U.S. Pat. No. 4,940,525(Dow Chemical Co.)

[0039] (b) Perfluorinated or partially fluorinated polymers containingaromatic rings such as those described in PCT/International PublicationNos. WO 95/08581, WO 95/08581 and WO 97/25369 (Ballard Power SystemsInc.), which have been functionalized with SO₃H, PO₂H₂, PO₃H₂, CH₂PO₃H₂,COOH, OSO₃H, OPO₂H₂, OPO₃H₂. Also included are radiation or chemicallygrafted perfluorinated polymers, in which a perfluorinated carbon chain,for example, PTFE, FEP, tetrafluoroethylene-ethylene (ETFE) copolymers,tetrafluoroethylene-perfluoroalkoxy (PFA) copolymers, poly (vinylfluoride) (PVF) and poly (vinylidene fluoride) (PVDF) is activated byradiation or chemical initiation in the presence of a monomer, such asstyrene, which can be functionalized to contain an ion exchange group.

[0040] (c) Fluorinated polymers such as those disclosed in EuropeanPatent Publication Nos. 0331321 and 0345964 (Imperial ChemicalIndustries plc) containing a polymeric chain with pendant saturatedcyclic groups and at least one ion exchange group which is linked to thepolymeric chain through the cyclic group.

[0041] (d) Aromatic polymers such as those disclosed in European PatentPublication No. 0574791 and U.S. Pat. No. 5,438,082 (Hoechst AG), forexample sulfonated polyaryletherketone. In addition, aromatic polymerssuch as polyether sulfones, which can be chemically grafted with apolymer with ion exchange functionality such as those disclosed inPCT/International Publication No. WO 94/16002 (Allied Signal Inc.).

[0042] (e) Nonfluorinated polymers include those disclosed in U.S. Pat.No. 5,468,574 (Dais Corporation), for example, hydrocarbons such asstyrene-(ethylene-butylene)-styrene,styrene-(ethylene-propylene)-styrene and acrylonitrile-butadiene-styrenecopolymers and terpolymers, in which the styrene components arefunctionalized with sulfonate, phosphoric and/or phosphonic groups.

[0043] (f) Nitrogen containing polymers including those disclosed inU.S. Pat. No. 5,599,639 (Hoechst Celanese Corporation), for example,polybenzimidazole alkyl sulfonic acid and polybenzimidazole alkyl, oraryl phosphonate.

[0044] (g) Any of the above polymers which have the ion exchange groupreplaced with a sulfonyl chloride (SO₂C₁) or sulfonyl fluoride (SO₂F)group, thereby rendering the polymers melt processable. The sulfonylfluoride polymers may form part of the precursors to the ion exchangemembrane or may be arrived at by subsequent modification of the ionexchange membrane. The sulfonyl halide moieties can be converted to asulfonic acid using conventional techniques such as, for example,hydrolysis.

[0045] In direct methanol fuel cells (DMFC), it is methanol that isoxidized at the anode during normal fuel cell operation, as follows:

CH₃OH+H₂O

CO₂+6H⁺+6e ⁻

[0046] Fuel starvation can also be a particular problem in themethanol-fuelled DMFC. The methanol can be blocked from theelectrocatalyst sites by the significant quantities of water that arepresent in the aqueous methanol fuel mixture and by the carbon dioxidegas that is generated by the electro-oxidation of the methanol.Consequently, the problems of cell reversal due to fuel starvation inthe anode structure, which are substantially identical to those outlinedfor the H₂-fuelled PEMFC, can be a problem in the DMFC. The use of afirst carbon-based component in the anode structure of the DMFC offers asignificant benefit. Just as in the H₂-fuelled PEMFC, the use of a firstcarbon-based component protects the vital carbon components in the anodefrom corrosion, by undergoing preferential corrosion, and also protectsthe membrane and catalyst layers from excessive dehydration andirreversible damage by helping to sustain the current density at lesspositive anode potentials. The use of a first carbon-based component inthe anode structure of the DMFC allows the MEA to provide a performancethat is not significantly reduced after a cell reversal incident.However, the problem of carbon corrosion in the direct methanol fuelledPEM fuel cell is not likely to be as great a problem as in the H₂-fuelcell due to the increased amount of water at the anode, and thus cellreversal current should be consumed in electrolysis reactions.

[0047] In a further aspect, an MEA comprises the present anodestructure.

[0048] In a still further aspect, a fuel cell comprises an MEAcomprising the present anode structure. In a yet further aspect, a fuelcell comprises the present anode structure.

[0049] While the present anode structures have been described for use insolid polymer fuel cells, such as the proton exchange membrane anddirect methanol fuel cells, it is anticipated that they would be usefulin other fuel cells, as well. In this regard, “fuel cell” generallyrefers to a fuel cell having an operating temperature below about 250°C. The present anode structures are preferred for acid electrolyte fuelcells, which are fuel cells comprising a liquid or solid acidelectrolyte, such as phosphoric acid, solid polymer electrolyte, anddirect methanol fuel cells. The present anode structures areparticularly preferred for solid polymer electrolyte fuel cells.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

[0050] Specific examples of a gas diffusion layer for use in conjunctionwith the present anode structure may be prepared in the followingmanner.

EXAMPLES

[0051] A particulate catalyst component, containing a first carbon-basedcomponent is provided by dispersing 30 weight parts of a high surfacearea carbon black (the first carbon-based component, which may be Blackpearls 2000 or PICACTIF CSO-D, both available from Cabot Carbon Ltd.,Stanlow, South Wirral, UK, or Norit A Supra, from Norit Americas Inc.,Atlanta, USA) and 100 weight parts of a 20 wt % platinum, 10 wt %ruthenium catalyst, supported on Cabot Vulcan XC72R (from JohnsonMatthey Inc, New Jersey, USA) in 1200 parts of demineralized water. Tothis is added 10 weight parts of polytetrafluoroethylene (PTFE) as adispersion in water (ICI Fluon GP1, 64 wt % solids suspension) and themixture heated and stirred to entrain the PTFE particles within thecarbon catalyst materials. The slurry is filtered to remove excess waterand re-dispersed in a 2% methyl cellulose solution, using a high shearmixer, to produce a smooth mixture.

[0052] The anode electrode may be prepared by applying a layer of thecarbon/catalyst slurry described above to a pre-teflonated (18% byweight ICI Fluon GP1), rigid conducting carbon fiber paper substrate(Toray TGP-H-090, available from Toray Industries Inc, Tokyo, Japan) atan electrode platinum loading of 0.4 mg/cm² of electrode geometric area.The dried electrode is heated to 375° C. in air to sinter the PTFE Asolution of perfluorosulfonic acid in the aqueous form as described inEuropean Patent Publication No. 0731520 is applied to the surface of thecatalyst layer to provide a proton conductive interface with theelectrocatalyst and to act as a water reservoir for the carbon corrosionprocess.

[0053] An electrode so prepared may form the anode of an MEA. Thecathode may be of the more conventional type, currently widely employedin the PEMFC. The foregoing comprise a conventional pre-teflonated rigidconducting carbon fiber paper substrate (Toray TGP-H-090, available fromToray Industries Inc, Tokyo, Japan) to which is applied a layer of a 40wt % platinum, catalyst, supported on Cabot Vulcan XC72R (from JohnsonMatthey Inc, New Jersey, USA), at an electrode platinum loading of 0.6mg/cm² of electrode geometric area. The catalyst layer material isprovided by dispersing 100 weight parts of a 40 wt % platinum catalyst,supported on carbon black (Johnson Matthey High-Spec 4000) in 30 partsof a 9.5% dispersion of Nafion EW1100 (E. I. DuPont de Nemours & Co.) inwater, prepared according to methods described in EPA 731,520. Theparticulate catalyst is dispersed using a high shear mixer to produce asmooth mixture and is then applied to the cathode substrate. Thecomplete MEA is fabricated by bonding the anode and the cathodeelectrodes (with the face of the electrode comprising the platinumcatalyst component adjacent to the membrane) to a Nafion 112 membrane(supplied by E. I. DuPont de Nemours, Fayetteville, N.C., USA)

[0054] The MEA thus formed may be tested in a cell reversal situationaccording to the following procedure. The MEA is conditioned prior tovoltage reversal by operating it normally at a current density of about0.5A/cm² and a temperature of approximately 75° C. Humidified hydrogenmay be used as fuel and humidified air as oxidant, both at 200 kPapressure. The stoichiometry of the reactants (that is, the ratio ofreactant supplied to reactant consumed in the generation of electricity)is 1.5 and 2.0 for the hydrogen and oxygen-containing air reactants,respectively. The output cell voltage as a function of current density(polarization data) is determined. After that, each cell is subjected toa voltage reversal test by flowing humidified nitrogen over the anode(instead of fuel) while forcing 10A current through the cell for aperiod of time long enough to cause some damage to a conventional anodewithout causing the extensive damage associated with large increases inthe anode potential (23 minutes has been found to be an appropriatelength of time) using a constant current power supply connected acrossthe fuel cell. During the voltage reversal, the cell voltage versus timeis recorded. Polarization data for each cell is obtained once the cellhas returned to normal stabilized operating conditions to determine theeffect of a single reversal episode on cell performance.

[0055] Each cell is then subjected to a second voltage reversal test ata 10A current. This time, however, the reversal current is interruptedfive times during the test period to observe the effect of repeatedreversals on the cells. After 5 minutes of operation in reversal, thecurrent is cycled on and off five times (20 seconds off and 10 secondson) after which the current is left on until a total “on” time of 23minutes has been reached. Following the second reversal test,polarization measurements of each cell are obtained.

[0056] The above procedure for cell testing can be used not only for thethree specific examples described, but also for examples falling withinthe scope of the present teachings. Furthermore, although the examplesdescribed above relate to a gas diffusion layer according to the presentteachings, it is within the ability of those skilled in the art tomodify the procedure to produce a gas diffusion substrate and/or acatalyst-coated membrane according to the present teachings.

[0057] In the foregoing examples and embodiments, the rate of carboncorrosion may be determined by appropriate adaptation of the followingprocedure which is suitable for a liquid acid electrolyte fuel cell suchas, for example, a phosphoric acid fuel cell. A complete cell isassembled by inserting an anode structure (previously weighed) of theinvention and a reference electrode (for example, a dynamic hydrogenreference electrode) into a liquid electrolyte. The cell was left untilthe test temperature is reached (for example, 180° C.) and for the opencircuit voltage (OCV) of the anode structure to stabilize. The cell wasactivated and as soon as the potential of the working electrode reached1 volt, current readings were taken over a given time period. The cellwas dismantled and the anode structure reweighed. The log (corrosioncurrent) was plotted against log (time) and extrapolated to 100 minutes.The corrosion rate is expressed as current per unit weight of carbon(μAmg⁻¹C) after 100 minutes at 1 volt. Data for the corrosion rates of anumber of carbons in phosphoric acid fuel cells may be found inCatalysis Today, 7 (1990) 113-137, which is incorporated herein byreference in its entirety. Although the actual carbon corrosion rateswill be dependent on the particular environment in which the anodestructure is placed, the relative rates of the various carbons willremain substantially similar.

[0058] One measure that can be taken as an indication of the corrosionresistance of carbon is provided by the BET surface area measured usingnitrogen, as this detects the microporosity and mesoporosity typicallyfound in amorphous carbon structures. For example, Vulcan XC72R,Shawinigan and graphitised Vulcan XC72R are typicalsemi-graphitic/graphitic carbon blacks employed in fuel cells. VulcanXC72R has a surface area of 228 m²g⁻¹. This contrasts with a surfacearea of 86 m²g⁻¹ for graphitised Vulcan XC72R. The much lower surfacearea as a result of the graphitization process reflects a loss in themore amorphous microporosity in Vulcan XC72R. The microporosity iscommonly defined as the surface area contained in the pores of diameterless than 2 nm. Shawinigan has a surface area of 55 m²g⁻¹, and BETanalysis indicates a low level of carbon microporosity available in thissupport for corrosion. This contrasts with the much higher BET surfacearea of, for example, Black Pearls 2000 (1536 m²g⁻¹) reflecting in thiscase a high degree of microporosity in this carbon black that cancorrode. Carbon blacks with BET surface areas in excess of 350m²g⁻¹,such as BP2000, could be employed as the first carbon material in thefirst carbon-based component in the anode structure of the PEMFC.

[0059] There are other carbons that also have high BET surface areas inexcess of 350 m²g⁻¹, such as those classified as activated carbons. Suchcarbons are usually derived from the carbonization of vegetable matter(typically wood, peat or coconut husks), in which the carbon isgenerally amorphous in character and there is a range of possible poresizes from micropores to larger mesopores and macropores. Typicalexamples of these activated carbons are those produced under the generaltrade name Norit (Norit Americas Inc., Atlanta, Ga., USA) and Pica(Pica, 92300 Levallois, France). Such carbons could also be employed asthe first carbon material in the first carbon-based component in theanode structure of the PEMFC.

[0060] Another indication of the corrosion resistance may bedemonstrated by the carbon inter-layer separation d₀₀₂ measured from thex-ray diffractograms. Synthetic graphite (substantially pure graphite)has a spacing of 3.36Å compared with 3.45Å for Vulcan XC72R(graphitised), 3.50Å for Shawinigan, and 3.64Å for Vulcan XC72R, withthe higher inter-layer separations reflecting the decreasing graphiticnature of the carbon and the decreasing order of corrosion resistance.Thus, a first carbon material with an inter-layer separation of greaterthat 3.65Å may be suitable for use in the first carbon-based componentof the present invention. However, many carbons that show poorresistance to corrosion (and therefore may be of use in the firstcarbon-based component of the present invention) are amorphous in natureand therefore no inter-layer separation measurement can be obtained.

[0061] It is also possible that the first carbon-based component maycomprise a first carbon material which intrinsically demonstrates areasonably high resistance to corrosion but which is treated in such amanner, for example, by coating with a proton-conducting electrolyte,that the formed first carbon-based component as a whole shows little orno resistance to corrosion.

[0062] While particular elements, embodiments and applications of thepresent invention have been shown and described, it will be understood,of course, that the invention is not limited thereto since modificationsmay be made by those skilled in the art without departing from the scopeof the present disclosure, particularly in light of the foregoingteachings.

What is claimed is:
 1. In a fuel cell anode structure comprising asubstrate, a first carbon-based component comprising a first carbonmaterial, and a second carbon component, said second carbon componentbeing substantially more resistant to corrosion during cell reversal atfuel cell operating temperatures than said first carbon-based component,the improvement comprising: said first carbon-based component havingsubstantially no resistance to corrosion during cell reversal at fuelcell operating temperatures and said first carbon material having a BETsurface area of at least 35 m²g⁻¹.
 2. The improved anode structure ofclaim 1 wherein said substrate is a gas diffusion layer.
 3. The improvedanode structure of claim 2 wherein said first carbon-based component isdisposed on said gas diffusion layer.
 4. The improved anode structure ofclaim 2 wherein said first carbon-based component is disposed withinsaid gas diffusion layer.
 5. The improved anode structure of claim 2wherein said first carbon-based component and said second carboncomponent are mixed and disposed on said gas diffusion layer.
 6. Theimproved anode structure of claim 2 wherein said first carbon-basedcomponent and said second carbon component are mixed and disposed withinsaid gas diffusion layer.
 7. The improved anode structure of claim 2wherein said first carbon based-component and said second carboncomponent are disposed in separate layers on said gas diffusion layer.8. The improved anode structure of claim 2 wherein said first carbonbased-component and said second carbon component are disposed inseparate layers within said gas diffusion layer.
 9. The improved anodestructure of claim 1 wherein said substrate is a solid polymerelectrolyte.
 10. The improved anode structure of claim 9 wherein saidfirst carbon-based component is disposed on said solid polymerelectrolyte.
 11. The improved anode structure of claim 9 wherein saidfirst carbon-based component is disposed within said solid polymerelectrolyte.
 12. The improved anode structure of claim 9 wherein saidfirst carbon-based component and said second carbon component are mixedand disposed on said solid polymer electrolyte.
 13. The improved anodestructure of claim 9 wherein said first carbon-based component and saidsecond carbon component are mixed and disposed within said solid polymerelectrolyte.
 14. The improved anode structure of claim 9 wherein saidfirst carbon-based component and said second carbon component aredisposed in separate layers on said solid polymer electrolyte.
 15. Theimproved anode structure of claim 9 wherein said first carbon-basedcomponent and said second carbon component are disposed in separatelayers within said solid polymer electrolyte.
 16. The improved anodestructure of claim 1 wherein the second carbon component acts as asupport for an electrocatalyst material.
 17. The improved anodestructure of claim 2, wherein said second carbon component is a carbonfill for said gas diffusion layer.
 18. A membrane electrode assemblycomprising the improved anode structure of claim 1, wherein saidmembrane electrode assembly is voltage reversal tolerant.
 19. A fuelcell comprising a membrane electrode assembly comprising the improvedanode structure of claim
 1. 20. A fuel cell comprising the improvedanode structure of claim
 1. 21. A method of improving tolerance of afuel cell to voltage reversal, the method comprising incorporating insaid fuel cell the improved anode structure of claim 1.